Optical transmission, in which an information signal is modulated onto an optical carrier, is widely employed in modern communications systems. In particular, many long-haul transmission networks employ single-mode optical fibres for the transmission of digital information at high bit rates, using one or more optical carriers, or wavelengths, over each fibre. Indeed, recent advances in optical technologies, including improvements in optical fibres, optical modulators, and the development of reliable and commercially practical optical amplifiers, have enabled the deployment of optical transmission paths capable of transporting on the order of 1 Tb/s over distances of thousands of kilometers, without the need for electronic regeneration. Individual optical channels in such systems typically carry information streams at rates of 2.5 Gb/s, 10 Gb/s, or even higher.
However, long-haul optical transmission systems are ultimately limited by distortion and degradation of the transmitted signals, arising primarily from dispersion (eg chromatic and polarisation mode dispersion), nonlinear transmission processes, and noise introduced by optical amplifiers. Any one of these processes alone can be overcome, or at least mitigated, by relatively simple means. For example, the effects of chromatic dispersion, which is a linear process, can be reversed by suitable optical equalisation means, such as by propagating dispersion-affected signals through equalisers having a corresponding inverse dispersion characteristic, such as a suitable length of dispersion compensating fibre (DCF). However, since DCF represents an additional signal propagation path having its own attenuation and nonlinear properties, there remains a limit to the amount of dispersion that can be compensated in this manner while maintaining adequate overall signal quality. Similarly, nonlinear effects may be reduced by limiting the total optical power within each fibre, while conversely the effects of optical noise are minimised by maintaining a high optical power level. As will be appreciated, therefore, it is not so much the individual contributions to signal degradation that ultimately limit transmission distance and/or capacity, but rather the interactions between the contributing processes, and the means and methods employed for their minimisation.
At the same time as optical transmission technologies have been advancing, corresponding improvements have been made in the electronic and digital technologies deployed in terminal equipment, such as transmitting, receiving and switching nodes. In the past, it has not been considered viable to perform significant signal processing within the electronic domain, when operating at the very high bit rates employed in long-haul optical transmission systems. However, with recent technological advances this is no longer the case. There has accordingly been interest in recent times in seeking to mitigate the effects of optical signal degradation processes, and in particular dispersion and nonlinear effects, within the electronic, rather than the optical, domain. Advantageously, such an approach is anticipated to enable a greater proportion of the available optical power budget to be effectively deployed in achieving greater transmission capacity and/or distance.
While some advances have been made in the development of electronic processing for the mitigation of optical signal degradation, certain features of the optoelectronic interface have continued to limit the net benefits available from this approach. In particular, the most robust, practical and cost-effective optical transmission systems employ optical intensity modulation at the transmitting end, in combination with direct detection at the receiving end. Direct detection systems are also known as “square law” detectors, because the relationship between the optical field incident on the detector and the resulting electrical output (eg photocurrent) follows a square law, photocurrent being proportional to optical intensity or power, rather than optical field amplitude.
While recent developments in optical modulation technology have improved the practicality of generating transmitted optical signals having properties that differ from those of conventional intensity modulated signals, there remain considerable advantages in employing direct detection systems. In particular, direct detection receivers are effectively wavelength independent within the usual optical transmission windows, and do not require additional optical components (such as local oscillator laser sources) and associated optical and/or electronic control that is typically associated with alternative coherent detection approaches.
It is believed that further improvements are possible in the methods and apparatus utilised for the generation and transmission of optical signals, particularly in direct detection systems, and it is accordingly an object of the present invention to address the need to realise such improvements.